One problem in lithography is knowing the position relationship between the camera image and the substrate position.
Present lithographic alignment systems achieve alignment by referencing a fiducial in the reticle to a position of the stage. This assumes that the camera remains stable after the alignment and doesn't drift due to the effect of environmental conditions on the lens system; and, as a result, it provides no assurance that the image doesn't move between alignments, creating a misalignment.
In the present invention, alignment is through the lens, with fiducials in the reticle being projected through the lens system and being aligned with sensors carried by the lens and using a reflected image plane, not the image plane on the stage or substrate.